It is effective to increase the numerical aperture of an objective lens and to shorten a laser wavelength in order to maximize the capacity of an optical disk. Also, in recent years, the development of a multilayer recordable optical disk has progressed. In multilayer recording, it is important to keep a laser beam from attenuating due to absorption or dispersion of the laser beam on a recording layer formed between a disk base member and a target recording layer in irradiating the laser beam onto the target recording layer via the disk base member. In view of this, there is proposed a technique of reducing unnecessary absorption and dispersion of a laser beam on a site other than a focusing spot by utilizing a nonlinear optical effect such as two-photon absorption.
Aberration by tilt is one of the drawbacks involved in maximizing the capacity of an optical disk. The word “tilt”, as used throughout the specification and the claims, means tilt of an optical axis of a laser beam with respect to a normal line to a surface of a substrate of an optical disk. If the numerical aperture of an objective lens is increased, and the laser wavelength is shortened, an influence of aberration due to tilt of an optical disk is increased. In some cases, a substantial thickness of a disk base member is increased if recording is attempted onto a layer proximate to the bottom of the optical disk in multilayer recording, with the result that an influence of aberration by tilt is significantly large. Such an aberration fails to obtain a clear focusing spot, and lowers reliability in recording/reproducing. Accordingly, in recording information of a large capacity onto an optical disk, it is essentially important to accurately detect tilt of the optical disk.
Aberration by tilt includes odd symmetrical aberration such as coma aberration and astigmatism. The following fact is known in an optical system of forming a focusing spot on a flat disk substrate such as an optical disk: if aberration has occurred in an incoming optical path, such an aberration may be cancelled in an outgoing optical path. Therefore, it is impossible to detect tilt of the optical disk simply by measuring aberration of a reflected beam from the focusing spot. This is one of the problems to be solved in tilt detection.
A first example of the conventional tilt detection is disclosed, for instance, in Japanese Unexamined Patent Publication No. 11-232677 (called as “D1”). In this example, after a signal from a detector is divided into two components in tangential directions of an optical disk, the signal components are differentially amplified to generate a tangential push-pull signal. Front and rear edge portions of a mark on a recording layer are detected with use of the tangential push-pull signal. Tilt of the optical disk in the tangential directions is detected based on a symmetrical property of a crest value of the tangential push-pull signal at the front and rear edge portions of the mark. In D1, similarly to the tangential push-pull signal, a radial push-pull signal is generated in radial directions of the optical disk, and tilt of the optical disk in radial directions is detected based on a symmetrical property of the radial push-pull signal at the front and rear edge portions of the mark.
A second example of the conventional tilt detection is disclosed, for instance, in Japanese Unexamined Patent Publication No. 2003-77158 (called as “D2”). D2 discloses a simpler method of detecting tilt of an optical disk in tangential directions. According to this method, a reproduction signal is inputted to a differentiating circuit, an output from the differentiating circuit is compared with a predetermined level by a comparator circuit, and a pulse width of the output from the comparator circuit is measured to perform tilt detection. Similarly to the first example, in the second example, front and rear edge portions of a mark on a recording layer are detected, and tilt of an optical disk is detected based on a symmetrical property regarding the front and rear edge portions.
A third example of the conventional tilt detection is disclosed, for instance, in Japanese Unexamined Patent Publication No. 2003-16680 (called as “D3”). In D3, tilt is detected in the following manner. A defocused state is created by adding an offset component to a focus control signal through application of an offset voltage to a defocus detection signal when an objective lens is focused. A tracking error signal detected in the defocused state is extracted as a tilt signal in a radial direction of an optical disk.
The aforementioned conventional optical disks and optical disk apparatuses have suffered from the following drawbacks.
In the first and the second conventional examples, a groove or a pit is required to be formed in tilt detection. If the techniques disclosed in D1 and D2 are applied to multilayer recording, diffraction or dispersion may occur due to the existence of the groove or the pit formed in each of the recording layers, which may unduly reduce the light amount to be received on the optical disk.
Further, in these conventional tilt detections, when an optical disk is tilted, a side robe is generated in skirt portions in a tilted direction of a beam spot and in a direction opposite to the tilted direction. The conventional techniques utilize a phenomenon that a crest value of a differentiated waveform of a push-pull signal or a reproduction signal is decreased due to generation of the side robes.
When the tilt angle is small, the magnification of the side robe is small. Accordingly, sufficient tilt detection sensitivity in a tangential direction of an optical disk cannot be expected. Also, since a laser beam does not propagate in a radial direction of an optical disk, tilt detection sensitivity in the radial direction is smaller than that in the tangential direction. Therefore, the S/N ratio of a detection output concerning the tilt detection is significantly small, which resultantly leads to a low precision in tilt detection.
The third conventional example has a drawback that recording/reproducing is impossible while the optical disk is in a defocused state. It takes quite a long time to create a defocused state by moving an objective lens for tilt detection, move the objective lens again to focus onto a target recording layer, and to read an address from the recording layer to confirm that the objective lens has returned to the target recording layer. This arrangement lacks real-time responsiveness in tilt detection.